Cylinder oil for lubricating between the cylinder and piston and system oil for lubricating and cooling other parts are used in crosshead diesel engines that are mainly used in large ships. The present invention relates to a system oil composition for crosshead diesel engines (hereinafter simply referred to as a system oil composition).
A system oil composition is used for lubrication of parts other than lubrication between the cylinder and piston (piston ring) in crosshead diesel engines. Examples of lubricated parts include the crankshaft and piston pin bearings, as well as cams and power take-off gears, and therefore sufficient viscosity, anti-wear, and anti-scuffing performance are required. Furthermore, the system oil composition is also used for cooling the backside of the piston combustion surface (called the undercrown), and therefore an extremely high oxidation stability and detergency are required.
In order to remove impurities, such as sludge resulting from deterioration of the lubricating oil, wear particles, water, and the like, system oil composition is normally purified with a centrifugal purifier or an oil purification device such as a variety of filters, strainers, or the like. In a centrifugal purifier, water is used as a medium, and therefore additive agents in the lubricating oil and water come into contact at a high temperature. Hydrolysis thus occurs easily, or sediments are easily formed due to the interaction of additive agents, making water tolerance necessary. Various efforts have thus been made in order to resolve these problems (see PTL 1 through 4).